Rinderpest vaccination and the incidence and development of trypanosomosis in cattle.

An investigation was made into whether recent vaccination of cattle with tissue culture rinderpest virus would cause immunosuppression and lead to more frequent or more severe infection with trypanosomes in animals grazing in tsetse-infested areas. Herds of cattle on Galana Ranch in Kenya were divided, with approximately half of each herd being vaccinated with tissue culture rinderpest virus strain Kabete 'O', while the rest remained unvaccinated. The herds were then exposed to the risk of natural infection with trypanosomes on the ranch. Three experiments were performed during different seasons. Infections with Trypanosoma congolense and Trypanosoma vivax were frequently detected but there was no evidence that vaccinated animals were more likely to acquire trypanosome infections or to show a more severe disease than unvaccinated cattle. It is concluded that tissue culture rinderpest vaccine does not cause immunosuppression and can safely be used in cattle likely to be exposed to tsetse flies and trypanosomosis.

The effect of trypanosomosis on pregnancy in trypanotolerant Orma Boran cattle.

A study was undertaken to investigate the influence of trypanosomosis on the outcome of pregnancy in trypanotolerant Orma Boran (Bos indicus) exposed to natural tsetse challenge in an area of Kenya infested predominantly with Glossina pallidipes. Of 73 pregnant Orma heifers, 58 (79.5%) produced live calves at term, 13 (17.8%) aborted and 2 (2.7%) died of trypanosomosis. Of the 71 surviving animals, 22 (31%) were infected with Trypanosoma vivax , 21 (29.6%) T. congolense, and 26 (36.6%) had mixed infections with T. vivax and T. congolense. These results suggest that in areas of high trypanosomosis risk reproductive function is affected even in trypanotolerant cattle, and that both T. vivax and T. congolense can be responsible for the abortions observed in the field. It is suggested that maintenance of pregnancy in the face of trypanosome challenge was dependent on individual variation among the Orma cattle, but as challenge increased beyond the limits of effectiveness of trypanotolerance, disruption of pregnancy occurred.

Estimation of heritability of susceptibility to infection with Theileria parva in the tick Rhipicephalus appendiculatus.

Heritability of susceptibility to infection with Theileria parva was estimated from full sib families of Rhipicephalus appendiculatus ticks. Male and female ticks of 2 stocks were mated singly. Nineteen full sib families of the Muguga stock and 17 full sib families of the Kiambu stock were obtained. Nymphae of these families were fed on cattle infected with T. parva so that the ticks became replete on days 16 and 17 after infection when the blood was parasitaemic with intraerythrocytic piroplasms. The T. parva infections were assessed in the resultant adult ticks of each full sib group and the abundance of infection, the number of salivary gland acini infected/tick, was found to be the most useful parameter for analysis. Estimates of heritability of the susceptibility to infection with T. parva for the Kiambu and the Muguga tick stocks were 0.24 and 0.26 respectively. Using only the data from ticks which fed on day 16, the heritability estimates were 0.39 for the Kiambu stock and 0.59 for the Muguga stock. These results indicate that tick lines of high or low susceptibility for T. parva infection could be produced through selection.

Failure of chemoprophylaxis against bovine trypanosomiasis on Galana Ranch in Kenya.

The duration of prophylaxis provided by 1 mg kg-1 bodyweight of homidium bromide was compared with that provided by 1 mg kg-1 bodyweight of isometamidium chloride in a 12 month field trial involving 90 Boran cattle exposed to trypanosome challenge on Galana Ranch in Kenya. Weekly trypanosome prevalences of over 30% were observed during 4 of the 12 months. During these periods of heavy challenge, parasites were detected 2-3 weeks after administration of both homidium bromide and isometamidium chloride. Both prophylactic drugs were administered, on a group basis, eight times over the 12 month trial and in addition individual infections were also treated with diminazene aceturate. Isometamidium chloride provided slightly longer periods of prophylaxis than homidium bromide, 28.4 days compared with 25.4 days. There was a highly significant difference in the productivity of the two groups during a period of poor grazing. 27% of the isometamidium chloride herd died from a severe wasting condition with substantial liver damage evident on post mortem. The condition was not observed in the homidium bromide herd. The surviving animals in the isometamidium chloride herd had a mean annual weight gain of 24 kg less than that recorded in the homidium bromide herd.

The economics of trypanotolerance on Galana Ranch

Trials on the control of animal trypanosomiasis have been undertaken on Galana Ranch for more than ten years; and the results have been presented in many arenas concentrating primarily on productivity and genetic aspects. There has not yet; however; been a comprehensive economic analysis of these trials. This paper address the issue of the relative profitability of using trypanotolerant orma cattle compared with faster-growing but more susceptivle improved Kenya Boran. The results of eight different trails; mostly of twelve months duration; carried out in two areas of Galana Ranch; Kilifi District; with varying trypanosomiasis challenge between 1981-1989; are presented here. All trails compare the performance of groups of Orma and improved Boran steers; in terms of weight gain and the cost of tsetse/trypanosomiasis control required to achieve those rates of growth. Five of the trials compared alternative drug regimes for trypanosomiasis control; and three involved tsetse fly control using insecticide-impregnated targets. In trails using trypanocides; the important determinant of relative profitability is the trade-off between weight gain and treatment cost; although there is variation between years; especially with Orma cattle. In those trials where the Galana weight gains are significantly higher than Orma; the extra income compensates for increased treatment expenditure. However; in those cases where there is no major weight gain difference; the requirement of the Orma for less drug treatments makes it the more profitable breed. Where targets have been the major control measure; costs are similar for the Galana and Orma animals that weight gain is therefore the major determinant of relative profitability

Susceptibility of African buffalo and Boran cattle to Trypanosoma congolense transmitted by Glossina morsitans centralis.

Four African buffalo (Syncerus caffer) and four Boran cattle (Bos indicus) were each exposed to the bites of 10 tsetse flies infected with Trypanosoma congolense. Although both groups of animals became infected, the buffalo showed no clinical signs of trypanosomiasis while the cattle suffered from the disease characterized by pronounced skin reactions, high parasitaemia and severe anaemia. The prepatent periods in the buffalo varied from 18 to 27 days in comparison with 11 to 14 days in the cattle. In the buffalo, skin reactions were only detectable by histological examination of skin biopsies, the peak of parasitaemia was at least a hundredfold below that in cattle and after 54 days parasites were no longer detected. In contrast, the cattle had a continuous high parasitaemia until they were treated with a trypanocidal drug 60 days after infection. Neutralizing antibody to metacyclic trypanosomes appeared in the buffalo during the prepatent period, 15-20 days after infection, whereas in cattle neutralizing antibody was not detected until 10 days after the first peak of the parasitaemia, 25-30 days after infection.

Homidium bromide as a chemoprophylactic for cattle trypanosomiasis in Kenya.

Homidium bromide was used in a strategic chemoprophylactic regime to control trypanosomiasis in Boran cattle in Kenya. Trypanosome infection rates in cattle receiving homidium bromide prophylaxis were compared with those in control cattle which received no prophylaxis but were treated with diminazene aceturate when infected. Homidium bromide was administered twice during the year after which no infections were detected for periods of nineteen weeks and seventeen weeks respectively. The drug sensitivity of the infecting trypanosomes is believed to be a major factor in determining the duration of prophylaxis.

Genetics and trypanotolerance.

Genetic resistance to disease and its use in domestic livestock usually ranks last, if at all, amongst preferred disease control measures - usually preceded by measures such as chemotherapy, vector control and vaccination. Thus, interest in genetic resistance is often a reflection of dissatisfaction with other control strategies, and the current emphasis on trypanotolerant cattle in Africa is just such a case. Eighty years of tsetse fly eradication programmes have had little impact on tsetse distribution, although recent research with odour baited targets impregnated with insecticide brings hope for the future. The search for a vaccine has proved more arduous than anticipated and the number of drugs available for therapy and prophylaxis is limited. In the search for alternative solutions to the problem of African trypanosomiasis, attention has recently focused on genetic resistance - a subject normally covered by immunologists or veterinarians(3-7). In this article, Rosemary Dolan discusses the concept from the geneticist's viewpoint.

Trypanotolerance in East African Orma Boran cattle.

Comparative studies on two types of large East African zebu (Bos indicus) Boran cattle, on a beef ranch in Kenya, have indicated that a Boran type bred by the Orma tribe has a superior response to tsetse fly challenge. The Orma Boran when compared with an improved Boran was found to have lower trypanosome infection rates and, when untreated, better control of anaemia and decreased mortality.